There is provided a stable optical element having a fine, uniform, and wide-ranging domain inversion structure in a ferroelectric crystal. This includes a plurality of domain inversions (101) formed on an MgO:LiNbO.sub.3 substrate (100), and a groove (102) formed on the substrate surface between the domain inversions (101). The depth T' of substantially all of the domain inversions (101) satisfies the relation T'